The present invention relates to apparatus for determining the alcohol content in human breath. It is known that the judgment in reaction time of individuals such as, for example, drivers of trucks, is decreased under the influence of alcohol, the decrease being proportional to the alcohol content in the blood. This alcohol content is normally determined by a blood test. The alcohol content in the blood increases to a maximum approximately 1 or 2 hours after the alcohol has been ingested and then decreases slowly. Since, during its circulation, the blood also passes through the lungs and there comes in contact with the air breathed by the individual, the breath of an individual who has had alcoholic drinks will also contain a small amount of alcohol roughly proportional to the alcohol content of the blood. It has therefore been found desirable to determine the alcohol content of the blood rapidly by means of apparatus into which an individual breathes. In known apparatus of this type, the alcohol in the individual's breath causes a chemical reaction which changes the color in the test tube into which the individual breathed, the length of the discoloration increasing with increasing alcohol content. This method is used by the traffic police, but is not particularly accurate.
Furthermore apparatus is known for determining the amount of combustible organic compounds in gas mixtures also containing oxygen. In this apparatus the content of combustible compounds is determined by measuring the heat of reaction resulting from the burning of the combustible compounds by means of a heating conductor. In a known embodiment, two heating conductors are electrically energized and have both a different catalytic effectiveness in the burning process and a different but temperature varying resistance. The two heating conductors are connected to neighboring arms of a wheatstone bridge circuit. Either both of the heating conductors are mounted within the chamber or only one is mounted therein while the other is mounted outside. In each case however the measurement depends on a comparison of the resistance in neighboring arms of a bridge circuit, the heat of reaction of the combustion either influencing both conductors in a different manner or else influencing only one of the conductors so that an unbalanced current proportional to the difference flows in the bridge circuit. In another known embodiment of this type, two heating conductors are arranged in the chamber, each wound on a core of ceramic material and having different catalytic effectiveness for the combustion process. Further associated with each core and connected to the neighboring arm of a wheatstone bridge circuit are two temperature sensitive resistance elements. The unbalanced current of the bridge circuit, which as mentioned above has an amplitude proportional to the content of combustible compounds, is used to generate either warning signals or to activate some switching process. However, the known apparatus is not particularly suitable for many practical applications since it is both complicated and requires a great deal of equipment and on the other hand only yields satisfactory results when all extraneous influences are carefully removed. These extraneous influences in particular include a uniform gas supply and an exact temperature control.